Thermal interface material on package

ABSTRACT

A packaged assembly is disclosed, including thermal interface material dispensed on an organic package and methods of manufacturing. The method includes dispensing a thermal interface material (TIM) on an electronic assembly. The method further includes removing volatile species of the TIM, prior to lid placement on the electronic assembly. The method further includes placing the lid on the TIM, over the electronic assembly. The method further includes pressing the lid onto the electronic assembly.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and, moreparticularly, to thermal interface materials dispensed on an organicpackage and methods of manufacturing.

BACKGROUND

Thermal management of multi-chip electronic packages is critical toideal performance of both single and multi-chip electronic packages.Currently, management of thermal performance in multi-chip electronicpackages is provided by encapsulating chips between a lid (e.g., heatspreader) and chip carrier using a thermal interface material (TIM) onthe chips. For example, a TIM is dispensed on the chip and a lid is thenpressed on the TIM to dissipate the heat generated by the chip, in thepackaged assembly.

Adhesion of TIM between the lid and chip interface needs to be optimizedin order to ensure adequate thermal performance. This is especiallyimportant due to increases in the chip size (e.g., higher than 20 mm)placed on an organic laminate. That is, due to the larger package, thestability and the adhesion of TIM undergo increased stresses due tothermal mismatch between the organic laminate and the chip. Thesestresses can result in delamination of TIM due to the bending of thepackage. Also, voiding phenomenon of the TIM decreases thermalperformance of the package, which is also directly related to lack ofcoverage and reduced adhesion.

SUMMARY

In an aspect of the invention, a method comprises dispensing a thermalinterface material (TIM) on an electronic assembly. The method furthercomprises removing volatile species of the TIM, prior to lid placementon the electronic assembly. The method further comprises placing the lidon the TIM, over the electronic assembly. The method further comprisespressing the lid onto the electronic assembly.

In an aspect of the invention, a method comprises removing organiccompounds from a laminate and chip. The method further comprisesdispensing a thermal interface material (TIM) on the chip. The methodfurther comprises applying an adhesive around a periphery of thelaminate. The method further comprises removing volatile species fromthe TIM. The method further comprises placing a lid on the laminate, incontact with the adhesive and the TIM. The method further comprisespressing the lid onto the adhesive and the TIM, at a predeterminedpressing force to form a packaged assembly. The method further comprisescuring the packaged assembly.

In an aspect of the invention, a method comprises: characterizing athermal interface material (TIM); quantifying a characterization ofvoiding of the TIM; determining process parameters to optimize adhesionand minimize voiding level of the TIM; validating a thermal performanceof the TIM using the steps of characterizing, quantifying anddetermining; and, if the validating does not meet thermal requirementsof the TIM, then reverting to the determining step to adjust the processparameters.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows a structure and respective processing steps in accordancewith aspects of the present invention;

FIG. 2 shows a process flow for manufacturing the assembly of FIG. 1, inaccordance with aspects of the present invention;

FIG. 3 shows a correlation of TIM voiding and outgassing (removal) ofvolatile species, in accordance with aspects of the present invention;

FIG. 4 shows an outgassing analysis at room temperature in accordancewith the present invention;

FIG. 5 shows a graph comparing shear strength of a lid to die, inaccordance with aspects of the present invention; and

FIG. 6 shows a methodology to improve TIM performance, over a range ofTIM products in accordance with aspects of the present invention.

DETAILED DESCRIPTION

The invention relates to semiconductor structures and, moreparticularly, to thermal interface materials (TIM) dispensed on anorganic package and methods of manufacturing. In more specificembodiments, the present invention relates to methods of increasing theadhesion (e.g., elimination of a voiding phenomenon) and coverage of aTIM to improve thermal performance of a packaged assembly.Advantageously, by implementing the processes of the present invention,it is possible to improve the coverage and adhesion of the TIM to thechip surface and mitigate the impact of very large chip surfaces.

The improved adhesion of the TIM can be provided by different mechanismsincluding, for example, removing (e.g., evaporating) volatile species ofthe TIM which, in turn, alters the viscosity and activates certainchemical compounds in the TIM to improve the adhesion properties. Forexample, the volatile species can be removed by evaporation through anextended delay process, low temperature heating process or vacuumoutgassing process. The methods of the present invention furthercomprise finding an operating range where a reduced viscosity, a lowervolatile content and a maximum adhesion range is maintained during aheat spreader (lid) attach operation of TIM materials.

Experimentally, different solutions have been attempted to reduce thevoiding phenomenon with TIM, to no avail. These methods include, forexample, (i) different profile cure; (ii) faster ramp up methods; (iii)slower ramp up methods; (iv) different dispense methods, e.g., airpressure vs. auger pump; (v) different dispense patterns, e.g., dot, X,multi-line etc.; (vi) applying different loads at the lid pressurestage; (vii) heating the module before applying the final lid pressure;and (vii) two stage lid pressure apply (pre-press). These differentprocesses, though, did not eliminate the voiding phenomenon on TIMmaterial. The present invention has provided a solution to the voidingproblem by improving the adhesion properties of the TIM, e.g., improveadhesion (2X), and increase thermal performance, using the methodsdescribed herein.

FIG. 1 shows a structure and respective processing steps in accordancewith aspects of the present invention. More specifically, the assembly10 includes a laminate 12 having, for example, one or more capacitors 14and a chip (e.g., processor or other electronics assemblies) 16 disposedthereon. It should be understood, though, that other components, whetherthey be active or passive components, can also be provided on thelaminate 12. The laminate 12 can be an organic laminate. A conventionalunderfill material (encapsulant material) 18 bonds the chip 16 to thelaminate 12. One of skill in the art should understand that theunderfill material 18 can protect sensitive electronic assembliesagainst damage and provide a strong mechanical bond between the chip 16and the laminate, e.g., (circuit board) 12. An adhesive 20 is placedaround a periphery of the laminate 12 and a TIM 22 is provided on thechip 16. In embodiments, the TIM 22 undergoes the processes of thepresent invention in order to improve adhesion to the chip 16. A lid 24is attached to the laminate 12 by use of the adhesive 20 and the TIM 16.In embodiments, the lid 24 protects the components on the laminate 12,as should be understood by those of skill in the art.

FIG. 2 shows a process flow for manufacturing the assembly of FIG. 1.For example, at step 200, the module undergoes an ashing process toremove any organic compounds from the laminate and chip. In embodiments,the ashing process can be a conventional plasma operation. At step 205,the TIM is dispensed on the chip, using known methods. For example, theTIM can be dispensed using an auger, air pressure, or other conventionalmethod. In embodiments, the TIM can be dispensed in different patterns,depending on the application and engineering designs, for example. Atstep 210, a sealband (adhesive) is placed on the laminate. The sealbandcan be placed around a periphery of the laminate to maintain an adequatebond between the laminate and a subsequently placed lid.

At step 215, volatile species (e.g., cyclic siloxanes and decyltrimethoxysilane) are removed from the TIM in accordance with aspects ofthe present invention. The removal of the volatile species (cyclicsiloxanes and decyl trimethoxysilane) increases the adhesion propertiesand thermal performance of the TIM by eliminating the voidingphenomenon. In embodiments, the removal of these volatile species can beachieved by, for example, (i) an extended delay process, (ii) lowtemperature heating or (iii) vacuum outgassing process.

By way of further explanation, in the extended delay process, the TIMremains at room temperature, e.g., about 21° C., for about 60 minutes,prior to lid placement. As shown in the graph of FIG. 4, for example,this process removes the volatile species and, in turn, improvesadhesion properties, eliminating the void phenomenon. It should beunderstood that other temperature ranges are contemplated by the presentinvention for different times, in order to remove the volatile species.For example, the room temperature can vary between 18° C. to 26° C., andowing to variations in humidity, room temperature can be about 20° C. to26° C., more preferably, 23° C. to 26° C. and even more preferably 20°C. to 23° C. In embodiments, a higher temperature and shorter timeperiod is contemplated by the present invention or, alternatively, alower temperature and a longer time period is also contemplated by thepresent invention. The overriding issue in any of these differentvariations is the removal of the volatile species prior to lidplacement.

Alternatively, the TIM can undergo a heat treatment to maximizeevaporation and surface reaction. In embodiments, the heat treatment ofthe present invention comprises subjecting the TIM to a temperature ofabout 45° C. to 55° C. for about 15-30 minutes in an oven, prior to lidplacement. In more preferred embodiments, the heat treatment isconducted at about 50° C. for about 20 minutes or less, prior to lidplacement. In even more preferred embodiments, the heat treatment isconducted at about 50° C. for about 15 minutes, prior to lid placement.The heat treatment of the present invention removes the volatile speciesand, in turn, eliminates the void phenomenon, improving adhesion andthermal performance properties of the TIM. This heating process wasfound to be more efficient with a maximal chemical surface area exposedin force air convection. That is, this heating process was faster thanthe extended delay process, e.g., improved throughput capacity.Moreover, the thermal performance of the package also increased usingthe processes of the present invention, compared to conventionalprocesses.

As a further alternative, outgassing of the volatile species is alsocontemplated by the present invention, using a vacuum chamber. Inembodiments, the outgassing processes can be provided at, e.g., about 23in mg for about 10 minutes.

As should be understood by those of skill in the art, the methods of thepresent invention eliminated TIM voiding and delamination of TIM onlarge die products. The methods of the present invention also work onboth single and multi chip modules. Moreover, in any of theseembodiments there was no yield impact within the complete range ofmaterial properties. Additionally, and advantageously, stress data,e.g., thermal aging and thermal and humidity exposure, shows improvementof thermal performance at the corners of the interface between the chipand the lid. For example, the heating processes of the present inventionincreased the thermal performance of the TIM at the corners of theinterface between the chip and the lid up to 10%, compared toconventionally processed packages.

Referring still to FIG. 2, at step 220, the lid is picked up and placedon the laminate. At step 225, the lid undergoes a pressing process toensure proper bond line thickness to the adhesive and the TIM. At step230, the packaged assembly undergoes a conventional curing process asrecommended by the TIM/adhesive supplier, e.g., ramp up, dwell and cooldown.

FIG. 3 shows a correlation of TIM voiding and outgassing (removal) ofvolatile species at 50° C., e.g., using the preheating process of thepresent invention. More specifically, FIG. 3 shows the gradual reductionin voiding, with improvement in adhesion occurring after about 10minutes. In fact, FIG. 3 shows that peripheral delamination disappearscompletely in the process of the present invention. As furtherdescribed, significant increase in outgassing of decyl trimethoxysilaneoccurred at 10 minutes into the 50° C. isothermal hold followed by asteady-state evolution throughout the remainder of the 50° C. hold. Aninitial bump in outgassing of the cyclic siloxanes was followed byprogressive drop with continued hold at 50° C.

More specifically, the top portion of FIG. 3 shows several sonograms300, 305, 310 and 315 of the TIM, representing the voiding phenomenon.These different sonograms 300, 305, 310 and 315 are correlated over timeand temperature to the removal of cyclic siloxanes and decyltrimethoxysilane. By way of example, sonogram 300 represents a voidingphenomenon of the TIM prior to heating. Sonogram 305 shows a slightreduction of the voiding phenomenon after 5 minutes at 50° C. Thisslight reduction is correlated with an outgassing (removal) of decyltrimethoxysilane. Sonogram 310 shows a further reduction of the voidingphenomenon after 10 minutes at 50° C. This further reduction iscorrelated with a further outgassing (removal) of decyltrimethoxysilane. Sonogram 315 shows the complete elimination of thevoiding phenomenon after 15 minutes at 50° C. This elimination iscorrelated with an outgassing of cyclic siloxanes tetramer. Accordingly,FIG. 3 demonstrates that the removal of decyl trimethoxysilane andcyclic siloxanes tetramer will eliminate the voiding phenomenon after 15minutes at 50° C.

FIG. 4 shows an outgassing analysis at room temperature in accordancewith the present invention. As shown in FIG. 4, after 60 minutes at roomtemperature there is no outgassing of cyclic siloxanes and decyltrimethoxysilane (as shown by Lines “A” and “B” respectively). Throughactual test data, the voiding phenomenon of the TIM was found to beeliminated at about 60 minutes at room temperature.

FIG. 5 shows a graph comparing shear strength of a lid to die for a lowviscosity TIM and a higher viscosity TIM, in accordance with aspects ofthe present invention. More specifically, the graph of FIG. 5demonstrates improved shear strength of the TIM for both incoming lowviscosity and high viscosity TIM products, compared to conventionalcurrent process of record (POR). As shown in FIG. 5, for example, TIMfor both incoming low viscosity and high viscosity TIM products thatwere heated to 50° C. for less than 20 minutes show shear strengthimprovement over untreated incoming low viscosity and high viscosity TIMproducts. In embodiments, this improvement was more than 2× improvement.Accordingly, improvement in adhesion was seen using the methods of thepresent invention regardless of the incoming viscosity of the TIMproducts.

FIG. 6 shows a methodology to improve TIM performance, over a range ofTIM products in accordance with aspects of the present invention.Specifically, at step 600, the characterization of the TIM is providedor determined. This characterization includes, for example, volatilecontent, temperature range for polymerization and cure, rheologicalbehavior and viscosity range. At step 605, quantification andcharacterization of voiding level and adhesion is provided (determined),for the material characterized at step 600. As step 610, processparameters to optimize adhesion and minimize voiding level are initiallydefined. These process parameters can include, for example, dispensemethod such as the dispensing equipment and pattern, dispense delay,pressure load apply during adhesive cure, and time and temperature rangefor preconditioning chemical prior to cure step. At step 615, thethermal performance can be validated. If the thermal performance is notwithin defined ranges, e.g., within required thermal performance, theprocess reverts to step 610 where the process parameters can be adjustedto optimize adhesion and minimize voiding level.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method, comprising: dispensing a thermalinterface material (TIM) on an electronic assembly comprising alaminate; placing an adhesive about a periphery of the laminate;removing volatile species of the TIM, prior to heat spreader placementon the electronic assembly; placing the heat spreader on the TIM, overthe electronic assembly; and pressing the lid onto laminate of theelectronic assembly and securing the heat spreader thereto by theadhesive, wherein the removal of the volatile species comprisessubjecting the TIM to an outgassing process, wherein the pressingincludes pressing the heat spreader onto the adhesive and the TIM, at apredetermined pressing force to form a packaged assembly and to ensure abond line thickness of adhesive.
 2. The method of claim 1, wherein thevolatile species comprises cyclic siloxanes and decyl trimethoxysilane.3. The method of claim 1, wherein the outgassing of the volatile speciesis provided at about 23 in mg for about 10 minutes.
 4. The method ofclaim 1, wherein the outgassing process is provided using a vacuumchamber.
 5. The method of claim 1, wherein the outgassing is provided ina vacuum chamber about 23 in mg for about 10 minutes.